1. Field of the Invention
This invention relates to the field of ceramics, and particularly to the field of low thermal conductivity Si.sub.3 N.sub.4 /ZrO.sub.2 ceramics.
2. Description of the Prior Art
Many different silicon nitride, Si.sub.3 N.sub.4, compositions have been investigated. One of these is the composition produced by the addition of zirconium oxide, ZrO.sub.2, to Si.sub.3 N.sub.4, to form Si.sub.3 N.sub.4 /ZrO.sub.2 composites. The Si.sub.3 N.sub.4 is a low expanding material, that is it has a low thermal expansion coefficient but relatively high thermal conductivity. ZrO.sub.2 has a relatively high thermal expansion coefficient but low thermal conductivity. However, when ZrO.sub.2 is added to Si.sub.3 N.sub.4, the thermal conductivity of the Si.sub.3 N.sub.4 is dramatically reduced, while the thermal expansion thereof is increased only to a moderate degree. The resulting Si.sub.3 N.sub.4 /ZrO.sub.2 composite ceramics are accordingly useful in those applications requiring a ceramic having a relatively low thermal expansion coefficient and thermal shock resistance of Si.sub.3 N.sub.4 materials, yet also having a low thermal conductivity. These applications include use in adiabatic Diesel engine components and radomes.
However, during fabrication of such Si.sub.3 N.sub.4 /ZrO.sub.2 composites by sintering at high temperatures, Zr-oxynitride is formed, which readily oxidizes at temperatures between 500.degree. C. and 1000.degree. C. to form monoclinic ZrO.sub.2 as a reaction product on the surface of the composite. The large molar volume change associated with this oxidation reaction produces large surface compressive stresses which causes surface spalling and rapid material degradation, particularly when the initial volume percent of the ZrO.sub.2 in admixture with the Si.sub.3 N.sub.4, exceeds about 10%.